What's more beautiful than one lady? It's TWO ladies! Why just have one girl when you can have two? (In an illustration, we mean.) There's something to seeing a couple of girls just hanging out, and there are slight variations in mood and atmosphere depending on whether they're about the same age or not. It's an important element to incorporate when drawing pairs of people!
Still working on that last point? Take a look through the illustrations below and see if you can use them as a reference!